Fiber reinforced composite parts are fabricated utilizing a variety of conventional techniques including resin transfer molding (RTM), PREPREG procedures, and preforming operations. These three techniques are discussed in turn.
RTM is a derivative of injection molding except that fluid (resin) is injected into a fibrous preform instead of an empty cavity mold. RTM, however, requires labor intensive and hence costly lay-up of the sheets of dry fiber onto a mold. After hand lay-up is completed, the mold is closed and resin is injected and allowed to cure. Another disadvantage is that uniform impregnation of the fiber workpiece is difficult to achieve. The learning curve associated with new geometries includes costly experimentation necessary to optimize processing variables and tool design to achieve void free uniform impregnation. Accordingly, complicated analysis and experimentation involving various configurations of the resin injection ports and vents is required to achieve uniform dispersion of resin through the fiber workpiece. Another problem with convention of RTM techniques is that resin gets caked on the tools, therefore, the tool surfaces must be thoroughly cleaned in order to prevent surface irregularities on successively formed parts. Still, RTM has advantages. It uses the lowest cost constitutive material possible (fiber and resin) and a high degree of geometric complexity and part integration is possible. Also, because injection pressures are relatively low, equipment costs are typically less than other methods.
Prepreg procedures involving hand lay-up of PREPREGs provide one advantage over RTM in that the problem of uniform dispersion of the resin is diminished somewhat. But, since the PREPREG must be cured in an autoclave, and since labor intensive lay-up of the fiber plies is still required, PREPREG techniques still possess many of the same inherent disadvantages. Furthermore, the PREPREG materials are expensive as compared to the raw resin and dry fiber used in RTM. Additionally, the uncured PREPREG fiber plies have certain specified shelf life and temperature limitations which add to the overall costs of manufacture. Moreover, the autoclave itself introduces additional costs as well as temperature and pressure control problems, and means are needed to prevent separation of the plies while they cure in the autoclave. Most importantly, however, PREPREG techniques add nothing over the RTM process to overcome the costly labor intensive lay-up of the plies of fiber material.
Preforming methods utilizing sizing or tackifiers partly eliminate the disadvantage of the expensive hand lay-up required in RTM and PREPREG fabrication techniques. But, as with PREPREG, there are inherent problems. Primarily, since the preform must be transferred to a conventional RTM tool for resin injection, preforming does not overcome the problem of nonuniform impregnation. Also, as with PREPREG procedure, the raw material costs are higher than the raw fiber and resin used in RTM. Finally, in conventional preforming operations, part geometry is often limited since the difficulty of maintaining preform material about the surface contours of the mold without undue wrinkling increases as the geometry becomes more complex.
One attempt to overcome the difficulty of maintaining the preform material about the contours of the mold surface utilized elastomeric sheets to prevent binding and wrinkling during the preforming process. A flat sheet of the preform material is placed between the two elastomeric sheets during forming and as the preform is shaped on the mold, the elastomeric sheets purportedly minimize fiber distortion and tears or rips in the material. Even using this technique, however, the preform must later be impregnated and all the problems of conventional RTM processes reappear. In forming hollow parts it is also known to use flexible expandable members to aid in conforming the resin injected fiber reinforced material to the interior of the hollow mold cavity. An inflatable bladder must also be used, however, and special configurations of the fiber material are required so that the material may move within the mold cavity without the need to preform the fiber material to the shape of the mold.
Therefore, these methods individually do not solve all the problems of the cost effective RTM method without introducing their own shortcomings. This is also true for other techniques such as thermoplastic automated tape layering or filament winding processes, and pultrusion. Unless a given fabrication system shows overall cost benefits to the user, it has little value as a practical innovation. Unfortunately, no known system singularly eliminates the labor intensive lay-up required by RTM and PREPREG procedures, the problems inherent in forming complex geometries, and the problem of nonuniform impregnation in fabricating a fully impregnated and formed part.